The present invention relates generally to systems and methods for precision analysis of gaseous systems, and more particularly to an all-solid-state continuous-wave laser system for mid-infrared (IR) absorption measurements of the carbon monoxide molecule.
In response to an increasing concern in recent years over the environmental impact of combustion emissions, manufacturers are creating new combustion equipment capable of extremely low emissions. Advanced gas turbines now available emit combustion exhaust having less than 10 ppm CO concentration, which lies near the lower detection limit of current sensor technology. To meet this inadequacy in sensor sensitivity, diode-laser-based sensors have been investigated to measure the very low concentrations of CO emissions characteristic of state of the art combustion equipment. Diode-laser-based sensors are characterized by high sensitivity, non-intrusiveness, and potentially continuous, real-time measurements of CO. With these attributes, diode-laser-based sensors may be ideally suited for incorporation into control systems to optimize combustion processes and minimize emissions.
Tunable diode-laser spectroscopy of the CO molecule has been reported on the fundamental band (near 4.6 μm) by using cryogenic lead-salt diode lasers (Varghese et al, “Collision Width Measurements of CO in Combustion Gases Using a Tunable Diode Laser,” J Quant Spectr Rad Transfer, 26, 339 (1981); Varghese et al, “Tunable Infrared Diode Laser Measurements of Line Strengths and Collision Widths of 12C16O at Room Temperature,” J Quant Spectr Rad Transfer, 24, 479 (1980); Miller et al, “Tunable Diode-Laser Measurement of Carbon Monoxide Concentration and Temperature in a Laminar Methane-Air Diffusion Flame,” Appl Opt 32, 6082-89 (1993)). Although successful, the implementation of in situ sensors based on lead-salt diodes has been limited due to operational complexity, the requirement of cryogenic cooling, and multimode operation. The transitions in the second-overtone band (1.3-2 μm region) of CO are about four orders of magnitude weaker than those in the fundamental band. By using an external cavity InGaAsP diode laser (ECDL) in the spectral range 6321-6680 cm-1, Mihalcea et al (Meas Sci Technol, 9, 327-338 (1998); Appl Opt, 36, 8745-52 (1997)) developed a diode laser absorption system for combustion emission measurements. The CO species was detected by identifying the R(13) line in the second-overtone band. In addition to the weakness of the lines, this spectral region includes strong interference from major species such as CO2 and H2O.
With the development of a room temperature continuous wave (cw) single mode InGaAsSb/AlGaAsSb diode laser that operates near 2.3 μm, Wang et al (Meas Sci Technol, 11, 1576-84 (2000); Appl Opt, 39, 5579-89 (2000)) addressed the first-overtone band of the CO molecule. Transition lines in the first-overtone are about two orders of magnitude stronger than those in the second-overtone band, and several transitions in the R branch are isolated from spectral interference with CO2 and H2O. Wang et al determined CO concentrations in the post-flame region (R(30) transition at 4343.81 cm−1) as well as the exhaust duct (R(15) transition at 4311.96 cm−1). For measurements in the post-flame zone, CO concentrations in rich flames were in good agreement with chemical equilibrium predictions. For measurements in the exhaust, the system achieved a detection limit of 1.5 ppm m at 470 K (50 kHz detection bandwidth, 50 sweep average, 0.1 s total measurement time). Wavelength modulation spectroscopy techniques were used to achieve a sensitivity of 0.1 ppm m (500 Hz detection bandwidth, 20 sweep average, 0.4 s total measurement time).
In attempting to detect CO in ambient air (typical abundance 150 ppb), Petrov et al (Optics Letters, 21, 86-88 (1996)) addressed the CO fundamental band at the R(6) transition near 2169 cm−1. Petrov et al investigated the use of a diode-pumped mid-IR difference-frequency mixing (DFM) source based on a periodically-poled LiNbO3 (PPLN) crystal to detect atmospheric CO, N2O and CO2. Their tunable mid-IR DFM source mixed a diode-pumped Nd:YAG laser (237 mW at 1064 nm single longitudinal mode) and a high power tapered GaAlAs amplifier pump seeded by a single frequency laser diode that allows fast frequency tuning by means of current modulation. With this system the detection sensitivity of 5 ppb M/√Hz was extrapolated based on rms noise measured in the 2f spectra under interference-free conditions.
The sensor system described herein solves or substantially reduces in importance inadequacies in prior art systems as described in the foregoing discussions by addressing mid-IR CO transitions in the fundamental band by means of DFM in a PPLN crystal using a novel compact Nd:YAG laser system that provides pump power, and the incorporation of an external-cavity diode laser (ECDL) that allows for frequency tuning. The invention is potentially applicable to numerous other molecules in the IR spectral region, such as as CO2, H2O, and C2H2.
The sensor described by the invention will allow real time monitoring of CO emissions from combustor systems such as gas turbine engine combustors. The invention may be incorporated into control systems to optimize combustion processes and minimize CO emissions by providing continuous, real-time CO measurements. The laser radiation is tunable and the laser can be tuned to the fundamental transitions of the CO molecule, resulting in estimated sensitivities of better than 1 ppm CO for a 1 meter path length in 1000 K gas.
It is therefore a principal object of the invention to provide a system for analysis of gaseous systems.
It is a further object of the invention to provide an improved system and method for precision analysis of flowing gaseous systems.
It is another object of the invention to provide an all-solid-state continuous-wave laser system for mid-infrared absorption measurements of the CO molecule.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.